1. Field of the Invention
The present invention relates to a color linear image sensing device and, more particularly, to a synchronous addition type color linear image sensing device comprising N (positive integer) arrays of sensors arranged in a vertical scanning direction, each sensor array including a plurality of sets of color filters disposed in a horizontal scanning direction, each color filter set being made of the same combination of color filters.
2. Description of the Related Art
One representative type of linear sensor is the so-called synchronous addition type linear sensor. This type of sensor has N (positive integer) arrays of sensors disposed in a vertical scanning direction, each array comprising numerous picture elements. A shift gate is furnished between sensor arrays. When an N-th sensor array reads a single line of text, the signal charges corresponding to that line are shifted to the (N-1)th sensor array through the sensor gate. At the same time, a drive system is activated to have the (N-1)th sensor array read the same single line of text. The operations are repeated until the added signal charges are transferred to a shift register provided independently of the N sensor arrays. The signal charges are then transferred from the shift register in the horizontal scanning direction (disclosed in Japanese Patent Laid-Open No. HEI/4-56461).
According to the synchronous addition type linear sensor as outlined above, each line of text is read by each of the sensor arrays and added up successively. This means that the sensitivity available with this sensor is N times as high as that of the sensor with no synchronous addition capability. The enhanced sensitivity is a major advantage of the synchronous addition type linear sensor.
What the inventors have devised now is a color linear image sensing device as an application of the synchronous addition type linear sensor. FIG. 4 is a plan view of a related art synchronous addition type color linear sensor developed by the inventors, indicating its major disadvantageous aspect. In FIG. 4, R stands for a red filter, G for a green filter and B for a blue filter. In each sensor array, the unit set of R, G and B occurs repeatedly in the horizontal scanning direction.
With the related art synchronous addition type color linear sensor of FIG. 4, the same points of an object are always scanned by the same color filters. This is because each of the sensor arrays comprises the same sets of color filters with no staggers of filter positions provided therebetween. In other words, there exist no discrepancies between the direction in which the signal charges are transferred and the direction in which the light spot advances for vertical scanning.
Given the above constitution, the related art synchronous addition type color linear sensor of FIG. 4 has a major disadvantage: it tends to develop a moire pattern. Suppose that a small image such as is encircled in FIG. 4 occurs in the red (R) and green (G) filter positions, as illustrated. In such a case, when scanned vertically, the image always occurs in the R and G filter positions. It follows that the blue component of the image is never detected while the signal charges corresponding to the image are being transferred to the shift register. This leads to the appearance of a spurious color pattern, i.e., moire.